1) Field of the Invention
The present invention relates to an optical filter device, and particularly to an optical filter device for splitting light which is to be used in a wavelength division multiplexing communication apparatus.
2) Discussion of Related Art
Recently, in order to realize information transmission of high speed and high density, wavelength division multiplexing communication in which light signals of different wavelengths are transmitted through a single optical fiber has been studied and developed. For a system for transmitting an image and a sound from a broadcasting station to a subscriber, particularly, a transmission procedure is under study in which a video signal is transmitted by light having a wavelength of 1.55 μm and an audio signal by light having a wavelength of 1.3 μm, in order to reduce the production cost of a line facility. In a wavelength division multiplexing light transmission module which is used in such a system, light must be split into two light components of different wavelengths. Conventionally, a dielectric multilayer film filter is therefore inserted into an optical waveguide layer.
Specifically, a groove of several tens of micrometers is formed in an optical waveguide layer, and such a filter is inserted into the groove. When the groove is narrowed, the insertion loss can be reduced. Therefore, the filter must also be formed so as to have a reduced thickness.
In the prior art, a dielectric film consisting of several tens of layers is grown on a polyimide film by the vacuum evaporation method. However, a process of forming a multilayer film by the vacuum evaporation method has a low mass productivity and hence there is a problem in that the production cost is very high.